The present invention relates to a tension mask for a color cathode-ray-tube and, more particularly, a tension mask having a border shield.
A conventional shadow mask type color cathode-ray-tube generally comprises an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color-emitting phosphors. In manufacturing the tube, a shadow mask, which is positioned with respect to the faceplate, is used in printing the screen array and, as such, defining the array borders. During tube operation, the shadow mask is precisely interposed between the gun and the screen to replicate the source positions during the screening process. The shadow mask effectively acts as a parallax barrier that shadows the screen and permits the transmitted portions of the electron beams to excite phosphor elements of the respective emissive color on the cathode-ray-tube screen.
In conventional tubes, the shadow mask is a domed thin sheet of metal capable of self-maintaining its configuration with the inner surface of the tube faceplate and is supported by a mask frame. Another group of masks commonly used in tubes are tension masks. Examples of tension type masks are tie bar and strand tension masks. Strand tension masks comprise a plurality of thin parallel strands that are stretched and welded to a rigid mask frame. The stretching of the strands provides the predetermined tension in the vertical dimension which is required to ensure that the apertures formed between the strands remain in alignment with the phosphor elements on the screen. In order to maintain the tension on the mask, the mask must be attached to a relatively massive frame.
Two different forms of attaching the strands to a frame can be found in conventional tubes. One form includes a border surrounding the central apertures of the mask which is welded to the frame. The solid border of the mask serves as an optical edge for forming the black surround of the matrix which in turn defines the borders of the screen array of the tube screen. A secondary purpose of the solid mask border is to provide an electron shield at the edge of the active scan region so as to reduce undesirable electron scattering during vertical overscan. The second form for attaching the strands includes attaching the ends of each individual mask strand to the frame. Both forms of tension masks have been found desirable for a number of reasons including aesthetic appearance of tubes with a face having limited or no curvature at all.
It has also been found desirable to make the mask and the mask frame from different materials to reduce the required mask tension and weight of the mask-frame assembly. In commercial tension mask tubes, solid borders of the mask are welded to the mask frame. The consequence of having a solid border of the mask welded to a frame when the mask and the frame have different thermal expansion coefficients is that deformation of the solid borders will occur along the mask-to-frame weld points during thermal processing of the tube, thereby permanently deforming the active portion of the mask. Such deformation has led the way to efforts to individually attach mask strands (or other etch mask portions) to the mask frame, wherein no solid mask border is attached to the frame. Unfortunately, individual attachment of mask strands has as also been problematic because the strands tend to displace from the pushing action of weld devices during welding. In addition to the process problems of attaching individual strands to the frame, the absence of solid mask borders is also not desirable because the borders serve as optical edges for forming and defining the black matrix surround and screen array and they also block stray electrons caused by the collision of the electron beam against the sides of the mask frame. Therefore, an invention is required that allows for individual attachment of mask strands to a mask frame without deformation, while also providing some terminating shielding which will serve as an optical edge for matrix and screening and a shield for stray electrons.
The present invention provides a tension mask for a cathode-ray-tube. The tension mask includes a frame with cantilevers attached to opposing sides of the frame and a plurality of spaced apart parallel strands extending between the cantilevers of the frame. A border shield is mounted over the strands along each edge of the frame cantilevers for subsequent welding technique. The border shield is incorporated to move into gripping relation with the respective strand ends, whereby the strands are held to the cantilevers in a predetermined space-apart relation.